Among various toxic anions dissolved in wastewater, a perchlorate ion (ClO4−) and a nitrate ion (NO3−) may be removed environmentally friendly by converting ClO4− and NO3− to non-toxic Cl− and N2, respectively, by a chemical reduction reaction as following Reaction Formula 1:

Various treatment technologies for removal of those anions have been reported so far, which can be classified into three general categories: (1) physical treatment, (2) bioremediation with microorganisms, and (3) catalytic reduction.
Each method has its own strengths and weaknesses. Physical separation methods such as ion exchange can allow fast and selective removal of anions from waste water containing other competing ions. However, a large volume of brine solution is required for the regeneration of saturated ion-exchange resin, which requires a secondary treatment process for decomposing the highly concentrated ClO4− and NO3− in the brine solution.
In contrast to the case of ion-exchange process, biological denitrification can permanently degrade ClO4− and NO3− into harmless chloride ion (Cl−) and nitrogen gas (N2), respectively. However, in the case of ClO4−, biological processes can be costly for the treatment of water containing low-concentration ClO4− because a highly reducing environment is required. Also, in the case of NO3−, the denitrifying bacteria require an organic carbon nutrient that must be added to the water being treated. The biological denitrification, either heterotrophic or autotrophic produces excessive biomass of the released bacterial cells as well as remnant carbon source, which requires intensive post-treatment process including several filtration steps and subsequent disinfection.
Similar to the biological denitrification, catalytic reduction in a liquid-phase by using H2 as a reducing agent can also permanently decompose ClO4− and NO3−. From a thermodynamic viewpoint, ClO4− is a strong oxidizing agent having a redox potential of +1.38 eV and hence can be permanently reduced to Cl−. However, the reduction is kinetically retarded because of its high activation energy (120 kJ/mol). Therefore, overcoming this energy barrier by using catalysts can permanently reduce ClO4− to Cl− in reductive conditions. Among these, Abu-Omar et al. reported the high catalytic activity of Re-based homogeneous catalysts in the presence of H3PO2 and organic sulfide as reducing agents (Abu-Omar, M. M. Inorg. Chem. 1995, 34, 6239-6240, Inorg. Chme. 1996, 35, 7751-7757, Angew. Chem. Int. Ed. 2000, 39, 4310-4313, Inorg. Chem. 2001, 40, 2185-2192, Inorg. Chem. 2004, 43, 4036-4050.). Re(V) complex can react relatively rapidly with ClO4− by an oxygen transfer reaction to form a Re(VII) complex, which can be reduced back to the Re(V) complex by the reducing agents. However, such a homogeneous catalyst with a soluble phosphorus or sulfur reducing agent is not readily compatible with water purification systems. As a heterogeneous version of Re catalysts, a supported Pd—Re bimetallic catalyst was also developed (Hurley, K. D. Environ. Sci. Technol. 2007, 41, 2044-2049). The catalyst, smartly combining the hydrogen activation ability of Pd and the oxygen transfer ability of Re, demonstrated reasonably fast reduction of ClO4− with H2 at acidic pH (pH<3). Nevertheless, all the Re-based catalysts still require significant improvements in catalyst activity and stability, especially at near-neutral pH. Wang et al. investigated ClO4− reduction with 78 different catalysts by using H2 as a reducing agent (Wang, D. M. Sep. Purif. Technol. 2008, 60, 14-21.), but none of the catalysts showed an appreciable reaction rate at near-neutral pH.
In the case of NO3−, it has been reported that noble metal catalysts (Pd, Pt and Rh) promoted with secondary metals (Cu, Sn, and In) can efficiently reduce NO3−, wherein PdCu bimetallic catalyst is widely accepted as the most active and selective combination (Jung, S. Environ. Sci. Technol. 2014, 48, 9651-9658, Kim, M. S. App. Catal. B: Environ. 2013. 142-143, 354-361, Palomares, A. E. J. Catal. 2004, 221, 62-66, Hörold, S. Environ. Technol. 1993, 14, 931-939, Hörold, S. Catal. Today, 1993, 17, 21-30.) It was proposed that the catalytic reaction takes place via a NO2− intermediate (Epron, F. J. Catal. 2001, 198, 309-318, Hörold, S. Catal. Today, 1993, 17, 21-30, Prüsse, U. J. Mol. Catal. A: Chem. 2001, 173, 313-328), wherein promoting metals such as Cu are responsible for the NO3− conversion into NO2− (Hörold, S. Environ. Technol. 1993, 14, 931-939, Epron, F. J. Catal. 2001, 198, 309-318, Hörold, S. Catal. Today, 1993, 17, 21-30, Prüsse, U. J. Mol. Catal. A: Chem. 2001, 173, 313-328). It has been reported that noble metals such as Pd and Pt do not present activity for NO3− reduction, but they can convert NO2− into N2 or NH4+ (Hörold, S. Environ. Technol. 1993, 14, 931-939, Epron, F. J. Catal. 2001, 198, 309-318, Hörold, S. Catal. Today, 1993, 17, 21-30, Prüsse, U. J. Mol. Catal. A: Chem. 2001, 173, 313-328). Noble metals also re-reduce the promoting metals by providing activated hydrogens (i.e., hydrogen spillover from Pd to Cu). As the reaction proceeded, one and two OH− ions should be generated as one N2 and NH4+ are generated, respectively, in order to maintain charge neutrality. However, so far most of catalytic studies were carried out using a batch reactor for fundamental investigations, which is not highly relevant for continuous treatment of waste water. Catalytic NO3− reduction with PdCu catalyst in a continuous-flow reactor showed that NO3− conversion is relatively low (Pintar, A. Catal. Today, 1999, 53, 35-50) and undesired NH4+ are too much formed (Pintar, A. Catal. Today, 1999, 53, 35-50, Palomares, A. E. Catal. Today, 2010, 149, 348-351, Palomares, A. E. Catal. Today, 2011, 172, 90-94), which indicated that a direct use of up-to-date PdCu catalysts is not yet highly practical. In order to overcome the limited catalytic activity and possible contamination of water with NH4+, Pintar et al. combined catalytic degradation with an ion-exchange process: NaCl brine solution was used for the regeneration of NO3−-saturated ion-exchange resin and the highly concentrated NO3− in the brine solution was catalytically reduced (Pintar, A. Chem. Eng. Sci., 2001, 56, 1551-1559). In principle, the integrated process can decrease the amount of waste brine produced and there is also lower risk that the side products of catalytic reduction such as NO2− and NH4+ can directly affect water quality.